Thermoset medium-voltage insulation compounds that have been used in 5,000 to 39,000 volt transmission and distribution cable, and higher voltage cables as well, have been based on cross-linked (also referred to as cured) polyethylene (PE) homopolymers, copolymers of ethylene and alpha olefins such as propylene, butene, or octene (EP, EB, EO), and terpolymers of ethylene, propylene, and a diene (EPDM). Any of those thermoset compounds are subject to in-service polymer oxidation or ionic contamination, which can result in a breakdown of their electrical performance.
Such polymer oxidation is referred to as electrochemical oxidation, and typically involves a two-step process. First, ground water often migrates through the cable jacket and the semiconductive insulation layer of the cable to contact the polymer. Second, electrical stresses at the polymer/water interface initiate a series of free-radical reactions, which lead to the decomposition of the water and the oxidation of polymer. Ionic contamination can result from (a) invasion of ionic species via ground-water ingress, as well as (b) concentration of residual ionic species in the thermoset compound.
Additives capable of reducing electrochemical oxidation by precipitation or complexation of undesirable ions may have beneficial impact on in-service cable performance. Medium-voltage insulation compounds, which are based on either ethylene-propylene copolymer or EPDM often contain, as a preferred ingredient, lead tetraoxide, Pb3O4. Lead tetraoxide has been shown to improve wet-electrical performance in these types of cables by preventing electrochemical oxidation. However, environmental concerns about the disposal of lead-containing chemicals or materials have resulted in efforts to develop replacements for lead tetraoxide in medium voltage insulation.
There is a need, therefore, for environmentally acceptable methods and systems to reduce or eliminate electrochemical oxidation in power cables.